1. Field of the Invention
This invention relates to mounting optical elements and more specifically to techniques for mounting optical elements formed from brittle materials in a high G environment without compromising optical performance.
2. Description of the Related Art
Electro-optical (EO) seekers are incorporated on guided missiles to acquire a target and guide the missile through impact. The seeker may include a number of optical elements such as a dome, window(s), lens element(s), mirror(s) and a detector. The optical elements must transmit, reflect or detect certain wavelengths, depending on the nature of the target's energy source(s) the missile's seeker is designed to acquire (i.e. infrared, ultraviolet, laser, visible). Because materials which have ideal properties for optimal optical performance are oftentimes brittle, this typically results in EO seeker designs specifying the use of brittle optical elements. Brittle materials are materials that are liable to fracture when subjected to stress beyond a failure point, they have little tendency to deform before fracture, i.e. no yield strength. Brittle materials often exhibit very low tensile strength as compared to their compressive strength, typically at least a factor of two lower. Material failure occurs in a statistical manner and is characterized by a probability of failure, which is based on the level of stress and the amount of material under that stress.
A common configuration for an optical lens is to utilize an annular flat on either, or both, of its surfaces, with a straight cylindrical peripheral diameter. This allows for straight-forward manufacture and simplifies the mounting of the element. Lenses are typically secured in place by use of adhesives or by retaining them by purely mechanical means. Optical element surfaces are typically designated as S1 for the forward facing surface that the energy first enters or reflects and S2, S3, S4 and so forth for the next succeeding surface in an optical assembly). For a single lens or window, therefore, S1 and S2 are all that is required to define the element surfaces. For the present discussion, ignoring the other elements, the projectile forward facing lens surface is S1 while the aft facing surface is S2.
A rocket powered missile launch produces acceleration during the boost phase that puts conventionally mounted optical elements into a plate bending condition producing compressive stress in their forward facing surfaces and tensile stress in their aft facing surfaces. Although tensile stress can be a cause of structural failure in brittle materials, because a rocket powered missile launch typically produces peak acceleration loading of less than 30 Gs, which induces relatively low tensile stress, the brittle optical elements usually have a high probability of survival.
The military is looking to extend EO seeker capability to gun-launched projectiles, which may be exposed to accelerations in excess of 20,000 Gs when fired, causing inertial loading, also referred to as set-back. Such high inertial loading produces tensile stresses within the optical elements which may be above the failure points of their corresponding materials resulting in a low probability of survival. As a significant portion of the induced stress can be attributed to the manner in which the element is mounted, an optical element mounting configuration that is effective in protecting the brittle optical elements from damage without degrading optical performance, during high G environment exposure or thereafter, is needed to enable guided gun-launched projectiles.
Stachiw et al. “Design Parameters for Germanium Windows Under Uniform Pressure Loading”, SPIE VOl. 131 Practical Infrared Optics (1978) pp. 57-72 investigated designs for brittle germanium windows exposed to uniform pressures of 10 to 20,000 psi. Stachiw noted that “Beveled edges tend to decrease the magnitude of tensile stress on low pressure face, but unfortunately their presence, as a rule, introduces high shear stresses in the conical bearing surface that tend to initiate shear cracks at that location. For this reason, windows with plane surfaces fabricated from brittle materials are generally limited to pressure less than 1000 psi, and preferably to less than 500 psi.” Stachiw concluded that “Only the windows with spherical faces can readily withstand hydrostatic pressures in excess of 1000 psi. The spherical surfaces are admirably suited for resisting external pressure since only compressive membrane stresses are generated in windows with such faces”. (pp. 58)
U.S. Pat. No. 6,212,989 entitled “High Pressure, High Temperature Window Assembly and Method of Making the Same” to Beyer discloses a window assembly including a truncated conical window, a truncated conical metal seal which circumferentially engages the window and a case to which the window and the seal are secured. Beyer's abstract states that “By initially seating the window assembly by means of mechanical pressing, then subjecting the window assembly to a series of successively higher transient combustion pressures, a window assembly can be fabricated which can withstand prolong service in a harsh environment such as that encountered in a large caliber artillery cannon or equivalent scientific and industrial applications.” Beyer's assembly pre-compresses the window to offset the tensile stress created on the aft surface when, for example, the artillery cannon is fired. This approach is not acceptable for optical elements which need to maintain position in order to achieve acceptable performance, as in EO seekers, due to the elements changing position during the seating in process.